LAUSANNE, Switzerland -- U.S. Secretary of State John Kerry will extend his stay in Switzerland to continue talks between six world powers and Iran over Tehran's controversial nuclear program.

U.S. State Department spokeswoman Marie Harf said on April 1 in the Swiss resort city of Lausanne that Kerry will stay "at least" until April 2 to continue negotiations.

She said the talks "continue to make progress," but that the sides "have not reached a political understanding."

Iranian Foreign Minister Mohammad Javad Zarif, for his part, urged officials from Britain, France, Germany, China, Russia, and the United States to "seize the moment" to realize an "opportunity which may not be repeated."

Zarif made his comments after an unusually short, 10-minute meeting with Kerry on the evening of April 1, RFE/RL's Radio Farda reported.

Zarif said Tehran had shown a "readiness to engage with dignity" during the negotiations.

The diplomats missed a self-imposed deadline of midnight on March 31 to agree on the outlines of a deal to curb Tehran's nuclear program in exchange for relief from painful economic sanctions.

The potential framework agreement is meant to pave the way for a comprehensive deal by June 30.

Such a deal would end a 12-year standoff over Iran's nuclear program, which Western states and Israel fear is aimed at creating nuclear weapons-production capability -- a charge Tehran denies.

In a live interview with Iranian state television, Araqchi said that if a joint statement was issued by the end of the day, he suggested it would contain no specifics.

He named sanctions relief and research and uranium-enrichment-related research and development as key stumbling blocks.

Early on April 1, Russian Foreign Minister Sergei Lavrov said that one "can say with confidence that the ministers have reached a general agreement on all key aspects of a final settlement of this issue."

"It will be put down in writing over the next few hours, maybe during the day," Lavrov added.

But U.S. diplomats close to the talks said it was "not true" that an agreement had been reached on all key issues.

British Foreign Secretary Philip Hammond later said, "I think we have a broad framework of understanding, but there are still some key issues that have to be worked through."

"Some of them are quite detailed and technical so there is still quite a lot of work to do but we are on it now and we'll keep going at it," he told the BBC.

A German diplomat was quoted as saying the talks had become stuck overnight on “several important issues.”

The unidentified diplomat said that progress was "noticeable” after technical experts worked all night, adding, "Nothing is decided but with goodwill an agreement [is possible]."

China called for compromises, saying that, if the talks get stuck, all efforts to resolve Iran's nuclear standoff will have been wasted.

"All parties must be prepared to meet each other halfway to reach an agreement," a Chinese Foreign Ministry statement issued to reporters in Lausanne said.

Chinese Foreign Minister Wang Yi left Lausanne on March 31 and French Foreign Minister Laurent Fabius left early on April 1, saying he would return when it was "useful" and when he was needed.

Lavrov also left on April 1.

Iran's chief nuclear negotiator, Araqchi, said Tehran hoped to conclude talks in Lausanne by the end of the day.

In a live interview with Iranian state television, Araqchi said that "until we have solutions to all problems we cannot have a comprehensive agreement," naming sanctions and research and development as key stumbling blocks.

Negotiators have been wrangling over the scope of uranium enrichment that Iran would be allowed to conduct, where stockpiles of enriched uranium should be stored, proposed limits on Iran's nuclear research and development, and the timing and conditions for the removal of sanctions.

About two hours after the late-night talks resumed, Iranian Foreign Minister Zarif emerged and told reporters that there had been "good progress."

"We have accomplished quite a bit, but people needed to get some rest and start over early in the morning. I hope that we can finalize the work on Wednesday [April 1]," Zarif told reporters.​

U.S. President Barack Obama was briefed on the status of the negotiations in Switzerland in a video conference late on March 31 with Secretary of State Kerry and other members of his national security team.

The five permanent UN Security Council nations and Germany are seeking verifiable curbs on Iran's nuclear program that ensure Tehran is not able to develop nuclear weapons.

An interim deal was reached in November 2013, but negotiators have missed two self-imposed deadlines for a comprehensive agreement since then.

Iran says its nuclear program is for purely peaceful purposes, mostly power generation, and it wants U.S., EU, and UN sanctions lifted swiftly.

"There will be no agreement if the sanctions issue cannot be resolved," Majid Takhteravanchi, an Iranian negotiator, told Iran's Fars news agency.

Iran Nuclear Talks Timeline: How We Got Here

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Officials from both sides have said the main sticking points were the removal of the UN sanctions, the rules for reimposing them if Tehran fails to adhere to the deal, and Iranian demands for the right to unfettered research and development into advanced nuclear centrifuges after an initial 10-year period covered by the potential agreement expires.

Officials said Iran was still demanding the lifting of all UN sanctions and a guarantee that that they cannot be reinstated without further negotiations. Officials have said Britain, France, and the United States want any removal of UN sanctions to be automatically reversible, but that Russia dislikes this because it would weaken its ability to use its Security Council veto power to influence further actions.

The six powers hope to ensure that for at least the next 10 years, Iran is at least one year away from being able to produce enough fissile material for an atomic weapon, and they want Iran's most sensitive nuclear work to be suspended for more than a decade.

In Jerusalem, Prime Minister Benjamin Netanyahu restated his concern that an agreement would not do enough to guarantee the safety of Israel, the only Middle Eastern nation that is believed to have a nuclear arsenal.

In a televised statement on April 1, Netanyahu called on international negotiators to seek a "better deal" with Tehran.

He said a worthwhile agreement would "significantly roll back" Iran's nuclear infrastructure and "link the eventual lifting of the restrictions on Iran's nuclear program to a change in Iran's behavior."

"Iran must stop its aggression in the region, stop its terrorism throughout the world, and stop its threats to annihilate Israel,” Netanyahu added. “That should be nonnegotiable and that's the deal that the world powers must insist upon."

With reporting by Hannah Kaviani of RFE/RL's Radio Farda in Lausanne, Reuters, dpa, AP, AFP, and TASS